Although I don't for a minute doubt the validity of this statement I'd like to offer my own slant on this particular peculiarity.

Firstly I'm not sure that guys particularly find lesbians per se a huge turn on, I believe it's more to do with the willing suspension of disbelief that men seem to be capable of when watching pornography coupled with the fact that lesbian acts in porn are rarely performed by genuine lesbians - I'll elaborate.

Men are turned on by pornography in general more than women as they are more prone to graphic visual stimulus than the fairer sex (ie: we're all pervs). Secondly the male pornography viewer is capable of allowing themselves to think the the women involved are actually enjoying themselves when in reality the female protagonists are, by and large, heterosexual. The fact that they are heterosexual and are usually seen later on in said porno copulating with a man can allow the male viewer to beleive (or at least fantasize) that he might at some point in his life be lucky enough to indulge in a similar menage a trois.

Conversely, male actors in gay movies are usually homosexual due in no small part to the disgust (largely nutured by society) instilled in a heterosexual male at the thought of indulging in sexual acts with another male.

The fact that the protagonists in a gay male movie are usually gay doesn't leave the female viewer with much of an option to imagine herself joining in.

I doubt very much that if a guy was to see two genuine lesbians 'getting it on' that he would be as excited about the idea as if he knew that they might really be interested in men.

Footnote: I conceed to girlotron that I am generalising somewhat (if only for the sake of brevity) but I still contest that the majority of female porn protagonists are straight, and to quote Phoebe from 'Friends' "And then there are bisexuals, but some just say they're kidding themselves".